The well-known “lost wax” investment casting process typically uses a refractory mold that is constructed by the buildup of successive layers of ceramic particles bonded with an inorganic binder on a fugitive (expendable) pattern material such as typically a wax, plastic and the like. The finished refractory mold is usually formed as a shell mold around a fugitive pattern.
The refractory shell mold residing on the fugitive pattern typically is subjected to a pattern removal operation, wherein the pattern is melted out of the shell mold. This operation leaves an empty “green” (unfired) refractory shell mold. The fugitive pattern materials typically have a thermal expansion rate many times greater than that of the refractory shell mold. If the fugitive pattern and refractory mold are heated uniformly, the fugitive pattern material will thermally expand more than the refractory mold. This will place the refractory shell mold under tension and will ultimately crack the shell mold. The avoidance of such shell mold cracking is why the fugitive pattern material removal has been typically conducted by methods such as a high pressure steam autoclaving or flash firing pattern removal. The removal of the fugitive pattern material by a high pressure steam autoclaving or flash firing is done to expose the outside of the refractory shell mold to high temperature. This high temperature causes heat to be conducted through the refractory shell mold more quickly so as to melt the surface of the pattern before the interior of the pattern thermally expands. This surface layer of melted pattern material extends all the way to where the pattern is exposed at the open part of the mold and accommodates the expanding pattern material inside the mold by forcing some of the liquid surface pattern material out of the mold opening. Such methods can still allow cracking of the refractory shell mold if the heat is not applied in a continuum along the surface of the fugitive pattern inside the mold. The connecting together of the refractory shell mold between adjacent patterns is one of the major causes of non-uniform heating of the pattern. That is, thicker regions of the refractory shell mold will hinder the application of heat to the pattern material and locally delay the melting of the surface of the pattern and disrupting of the continuum. This prevents the passage of surface liquid pattern material from a thinner mold region more remote from the mold opening than the thicker mold region. Such prevention of the passage of surface liquid pattern material causes a buildup of pattern pressure in the remote thinner mold region due to the thermal expansion of the pattern material and can lead to mold cracking. These problems require the use of a mold strong enough (e.g. thick enough) to resist the expansion pressure of the pattern material and often require the use of supplemental holes or vents through the mold to relieve pressure from unconnected expanding patterns. Stronger or thicker molds as well as the venting method are undesirable as they increase processing costs.
A plurality of the green refractory shell molds (sans patterns) then typically are loaded into a batch or continuous oven heated by combustion of gas or oil and heated to a temperature of 1600° F. to 2000° F. Alternatively, the mold may be heated by a method of copending patent application Ser. No. 10/241,819 filed Sep. 10, 2002, of common assignee herewith, which describes the heating of a mold with or without surrounded mold support sand. The heated refractory molds are removed from the oven and molten metal or alloy is cast into them.
The trend in investment casting is to make the refractory shell mold as thin as possible to reduce the cost of the mold as described above. The use of thin shell molds has required the use of support media to prevent mold failure as described by Chandley et. al. U.S. Pat. No. 5,069,271. The '271 patent discloses the use of bonded ceramic shell molds made as thin as possible such as less than 0.12 inch in thickness. Unbonded support particulate media is compacted around the thin hot refractory shell mold after it is removed from the preheating oven. The unbonded support media acts to resist the stresses applied to the shell mold during casting so as to prevent mold failure.
Thin shell molds however, are more prone to cracking during the pattern removal operation, such as the high pressure steam autoclave or flash fire pattern removal operation mentioned above, wherein the pattern is melted out of the shell mold.
The present invention provides a method of removing a fugitive pattern from a bonded refractory mold in a manner that reduces cracking of the mold.